Single sensor motor vehicle velocity detector

ABSTRACT

In this detector, the sinusoidally varying signal voltage induced in an inductive sense loop by a ferromagnetic motor vehicle passing over it is adjusted by a low pass filter having a 6 db. per octave high frequency roll off characteristic in order to obtain a signal waveform of essentially uniform amplitude regardless of vehicle velocity. The frequency of the initial portion of the induced signal, which is proportional to the vehicle velocity, is measured by sequentially detecting two successive peak transitions of the filtered signal and measuring the time interval therebetween.

United States Patent [is] 3,656,959 Overstreet [451 Apr. 11, 1972 [541SINGLE SENSOR MOTOR VEHICLE OTHER PUBLICATIONS VELOCITY DETECTORMeasuring Projectile Velocities Electronics lndustries- Oc- [72]Inventor: Scott M. Overstreet, Los Altos, Calif. tober, 1943, PP- 196[73] Assignee: Sylvania Electric Products, llnc. p Examiner Mihae] JLynch [22] Filed. May 25 1970 Attorney-Norman J. OMalley, Russell A.Cannon and John F. Lawler [2]] Appl. No.: 40g247 V [57] ABSTRACT [52]US. Cl 324/173, 324/179, 340/38 L In this detector, the sinusoidallyvarying signal voltage in- [51] Int. Cl...... ..G0lp 3/54 duced in aninductive sense loop by a ferromagnetic motor [58] Field of Search..324/ l 79, 173, 41; 340/38 L vehicle passing over it is adjusted by alow pass filter having a 6 db. per octave high frequency roll offcharacteristic in order [56] References Cited to obtain a signalwaveform of essentially uniform amplitude regardless of vehiclevelocity. The frequency of the initial por- UNITED STATES PATENTS tionof the induced signal, which is proportional to the vehicle velocity, ismeasured by sequentially detecting two successive 3,430,221 2/1969Barrlnger ..340/38 L P transitions of the filtered Signal and measuringthe time FOREIGN PATENTS 0R APPLICATIONS Interval therebetween- 829,7187/1938' France ..340/38 L 3 Claims, 6 Drawing Figures 25 3! 7. r 1 l IF34 l i START STOP 3 l FREQUENCY FULL-WAVE l PEAK Z RAMP SAMPLE HOLDSELECTlVE GATE CONTROL NETWORK RECTIFIER DETECTOR I CIRCUIT GENERATOR cmun' 1 13 I s? I -23 I HRE H 1 I38'\ H I I i I I DETECTOR t 7 RESET I I17 DELAY 54 1- I 5% 6 W I-\ 3 k r l I v J L l-\ 9 L 7 UTlLlZATlONAPPARATUS PATENTEDAPRHIQI? sum 1 UF3 5 mm M56 8 A "655128 3 mv h ww32525 29:37:;

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, SCOTT M. OVERSTREET M/QW AGE NT SINGLE SENSOR MOTOR VEHICLE VELOCITYDETECTOR BACKGROUND OF THE INVENTION a technique for detecting vehiclevelocity is to measure the time forthe vehicle to pass between twostationary sensors spaced a fixed distance apart. Principaldisadvantages of such a system are that two sensors are required tomeasure the velocity of a moving vehicle and that closely spacedvehicles cause the system to produce inaccurate readings. An object ofthis in- 4 vention is the provision of a vehicle velocity detectoremploying only a single passive sensor.

SUMMARY OF INVENTION In accordance with this invention, the frequency ofthe initial portion of the signal voltage induced in an inductive senseloop by a motor vehicle passing over it is measured to produce anindication of the vehicle velocity.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic block diagram ofapreferred embodiment of this invention;

FIGS. 2A-2G are waveforms useful in explaining the opera-- tion of thisinvention;

FIG. 3 is a curve representing the passband characteristic of thefrequency selective network in FIG. 1;

FIG. 4 is a block diagram of the frequency selective network in FIG. 1;

FIG. Sis a circuit and block diagram of the peak detector in FIG. 1; and

FIG. 6 is a block diagram of an alternate embodiment of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIG. 1, a systemembodying this invention comprises aninductive loop or sense coil 11, afrequency T selective network 12, rectifier 13, detection circuit 14,measuring circuit 15, and utilization circuit 16. Loop 11 is located inone of the trafic lanes 18 and 19 of a road. The loop is preferably alarge loop comprising several turns of electrically conductive wire andhaving a length L in the direction of vehicle travel that isapproximately one half the length of the shortest vehicle whose speed isto be detected. By way of example, in an embodiment of this inventionthat was built and tested the loop 1 1 comprised three turns of No. 14insulated wire and had a length L of 6 feet and width W of 7 feet. Ifincreased sensitivity is required, the loop may be made of more turns ofwire.

When a magnetic body such as a motor vehicle 21 passes over the loop, asignal voltage represented by curve 22 in FIG. 2A is induced in the coiland is applied on lines 23 to the frequency selective network 12. Thissignal voltage essentially comprises one and one-half cycles of lowfrequency damped oscillations, the initial portion of which is producedby the front of the vehicle passing over the loop. The high frequencyvariations in the low frequency signal are believed to be caused byrotary motion induced in the vehicle when the engine is running andvertical vibratory motion of the vehicle as it traverses the loop. Ithas been discovered that the low frequency of the initial portion of theloop signal voltage (e.g., during the time interval t, to t in FIG. 2A)is directly proportional to the vehicle velocity. Thus, the time periodT in FIG. 2A decreases as the vehicle velocity increases.

It has been determined that the amplitude of the loop voltage is alsoproportional to the vehicle velocity. More particularly, the amplitudeof the loop voltage increases 6 db each time the vehicle velocity isdoubled. This means that the amplitude of the loop signal voltage onlines 23 will vary approximately 36 db for vehicle velocities between 2and I00 miles per hour. The amplitude of the loop signal voltage;however, is also a function of the height of the undercarriage of thevehicle above the ground. Thus, the signal amplitude is not an absoluteindication of vehicle velocity. Also, vehicles traveling in lane 19 mayinduce signal voltages of sufficient amplitude (i.e., within the 36 dbdynamic range) in inductive loop 11 to produce false indications ofmotor vehicles traveling in the associated lane 18 unless the amplitudedifference due to the physical separation of the two lanes is maintainedby using the frequency selective network.

In accordance with this invention, the loop signal voltage on lines 23is coupled through a frequency selective network 12 having a passbandcharacteristic such as that represented by the curve 24 in FIG. 3 forproducing on line 25 a signal voltage having an amplitude that isrelatively constant and independent of vehicle velocity. The passbandcharacteristic of network 12 may, by way of example, have a breakfrequency of 0.25 Hz which corresponds to a vehicle velocity ofapproximately 5 miles per hour and a roll off of 6 db per octave.Network 12 may comprise a low pass filter 26 having the frequencyresponse represented by curve 24 and an amplifier 27 (see FIG. 4).Alternatively, filter 26 may be a low frequency bandpass filter having a6 db per octave roll off. Also, network 12 may comprise an amplifierhaving a gain response characteristic similar to that represented bycurve 24. If the loop signal voltage on lines 23 is of sufiicientamplitude, the network 12 may comprise a single low pass or lowfrequency bandpass filter. The waveforms in FIGS. 2A and 28 representthe loop signal voltage before and after filtering by network 12,respectively, for a vehicle traversing the loop at 10 miles per hour.Reference to these waveforms reveals that the high frequency noisecomponents in the loop signal voltage are also removed by the filteringaction of network 12.

Rectifier 13 is preferably a full wave rectifier for converting thesinusoidally varying signal voltage (see curve 30, FIG. 28) on line '25to a signal voltage on line 31 wherein all of the signal variations areof the same polarity (see curve 32 FIG. 3C).

Circuit 14 comprises peak detector circuit 33, threshold detectorcircuit 35, and gate circuit 36. Detector 33 is responsive to the signalvoltage 32 on line 31 for producing on line 34 the pulses 37 and 38indicating the occurrence of the peak transitions of curve 32 at timesand t respectively. Referring now to FIG. 5, detector 33 comprises acomparator 41 having first and second inputs 42 and 43 connected throughdiodes 44 and 45, respectively, to line 31. A third diode 46 isconnected in parallel with diode 45 in the opposite polarity. A storagecapacitor 47 is connected between the second input 43 of the comparatorand a ground reference potential.

Measuring circuit 15 comprises start-stop control circuit 51, rampgenerator 52, sample hold circuit 53 and reset delay circuit 54. Controlcircuit 51 is responsive to the successive pulses 37 and 38 passed bygate 36 for causing generator 52 to initiate generation of the linearramp voltage represented by curve 55 in FIG. 2E. Circuit 53 isresponsive to the output pulse 38' produced on line 56 by the controlcircuit at time t for sampling and holding a voltage corresponding tothe amplitude of the ramp voltage 55. Delay circuit 54 is responsive tothe pulse 38' for producing a control pulse 57 at time t see 7 FIG. 26,for resetting the ramp generator.

During quiescent operation when the voltage on line 31 is less than thethresholdlevel V of detector 35, see FIG. 2C, this circuit 35 operatesin a first conduction state to produce an output voltage that maintainsgate 36 closed. Consider now that a motor vehicle 21 traveling at avelocity of 10 miles per hour traverses the loop 11 causing network 12to produce on line 25 the signal voltage represented by curve 30 in FIG.2B. When the magnitude of the rectified sigial voltage 32 exceeds thethreshold level V at time detector 35 changes conduction states toproduce an output voltage that opens gate 36 until the signal voltagefalls below this threshold level at 2,. As the signal voltage 32 becomesmore positive, diodes 44 and 45 both conduct whereas diode 46 is reversebiased and cut off (see FIG. This causes capacitor 47 to store a chargecorresponding to the input signal voltage on line 31. Since the samesignal voltage is applied to both of the input terminals of thecomparator, the output thereof is constant. When the signal voltage 32falls below its peak value at time i diode 45 is reverse biased and cutoff by the charge stored on capacitor 47 which maintains the voltage ontenninal 43 essentially constant. Diode 44, however, continues toconduct causing the signal voltage on terminal 42 to decrease. When thedifference in potential between terminals 42 and 43 exceed a prescribedthreshold level, the comparator produces an output pulse 37 indicatingthe occurrence of the peak voltage transition at time t;,. Controlcircuit 51 is responsive to the pulse 37 for producing the pulse 37 online 59 which causes generator 52 to produce the linear ramp voltage 55.

In a similar manner, circuit 14 is responsive to the signal voltage 32between times 1 and 2,, for producing the pulse 38 indicating theoccurrence of the peak transition at time t,. Control circuit 51 isresponsive to the pulse 38 for producing the control pulse 38' on line56 at time t,. Circuit 53 is responsive to the control pulse 38 forsampling and holding the present value of the ramp voltage which is anindication of the velocity of the motor vehicle and is applied toutilization apparatus l6. Delay circuit 54 is also responsive to thecontrol pulse 38 for producing the delayed control pulse 57 at time twhich resets the ramp generator.

Referring now to FIG. 6, an alternate embodiment of this inventioncomprises a loop 11', frequency selective network 12', detectioncircuits 61 and 62, and measuring circuit Similar components in FIGS. 1and 6 are represented by primed reference characters in the latter.

Detection circuit 61 is operative for detecting one transition of thesignal voltage 30 on line 25'. Detection circuit 62 is operative fordetecting a transition of the signal voltage on line 25 occurringsubsequent to the one transition. Circuit 61 comprises a detector 64, athreshold detector 65 and a gate circuit 66. Detector 64 may be: acircuit similar to that illustrated in FIG. 5 for detecting theoccurrence of a positive or negative (i) peak of the signal voltage 30at time t, or t a circuit for detecting a change in the polarity of theslope of the signal voltage 30 at times 1 and t,; or a circuit fordetecting a zero crossing transition of the signal voltage 30 at times tand t Detector 65 comprises circuitry for producing on line 67 a controlvoltage that opens gate 66 when the absolute value of the signal voltage30 is greater than a prescribed threshold level V (see FIG. 2B).Detector 65 is preferably operative for opening gate 66 only during theone-half cycle of the signal voltage 30 between time t, and t Detectioncircuit 62 comprises multivibrator 70, gate circuit 71 and zero crossingdetector 72. Multivibrator 70 is responsive to the output of gate 66 forchanging operating states to open gate 71 for a prescribed time intervalto pass the signal voltage on line 25 to the zero crossing detector. Theprescribed time interval that gate 71 is open is preferably equal toapproximately T/2 for the waveform corresponding to a vehicle travelingat the slowest velocity to be measured. The multivibrator 70 may, by wayof example, be a one-shot multivibrator.

Measuring circuit 15' comprises a start-stop control circuit 73receiving the output of gate 66. The operation of control circuits 51and 73 are similar.

In operation, when the magnitude of the signal voltage 30 on line 25'exceeds the threshold level V at time detector 65 changes operatingstates to produce on line 67 an output that opens gate 66. When thesignal voltage 30 reaches its peak value at time I detector 64 producesan output pulse 77 which is passed by gate 66. Multivibrator 70 isresponsive to this control pulse for opening gate 71 to pass the signalvoltage 30 on line 25'. Control circuit 73 is also responsive to thepulse 77 for causing generator 52' to initiate generation of a rampvoltage. When the signal voltage 30 crosses the zero volt referencelevel at t,,, detector 72 produces the control pulse 78 which causescircuit 53' to sample and hold the peak value of the ramp voltage outputof generator 52 at time I, which is proportional to the frequency of theloop signal 30 and the vehicle velocity. Delay circuit 54' is responsiveto the control pulse 78 for producing at time i the delayed controlpulse 78 which resets generator 52'.

What is claimed is: 1. Apparatus for detecting the velocity of a movingvehicle comprising a single passive sensor comprising an inductive loopof electrically conductive wire oriented with respect to a roadway suchthat a vehicle moving in a prescribed orientation with respect to saidloop induces a sinusoidally varying voltage in said loop, means formeasuring the frequency of the initial portion of said induced signalvoltage for producing an indication of the vehicle velocity, comprisingfirst means for detecting the occurrence of one transition of saidinduced signal voltage, second means for detecting the occurrence of atransition of said induced signal voltage occurring subsequent to saidone transition, and means for measuring the time interval between theoccurrence of said one and said subsequent transition for producing anindication of the vehicle velocity, frequency selective means forcoupling the induced signal voltage in said loop to said first andsecond detecting means, said frequency selective means having a lowfrequency pass characteristic with a high frequency roll off ofapproximately 6 db per octave, and means for converting the sinusoidalvariations of the induced signal voltage from said frequency selectivemeans to one polarity with respect to a reference potential, said firstand second detecting means comprising third means responsive to theoutput of said converting means for detecting the time of occurrence ofthe peak values of consecutive variations of signal voltage. 2.Apparatus according to claim 1 wherein said time interval measuringmeans comprises a generator responsive to one output of said thirddetecting means for initiating generation of a linear ramp voltage, and

a circuit responsive to a subsequent output of said third detectingmeans for sampling and holding the magnitude of the ramp voltage, theamplitude of the ramp voltage being a measure of the vehicle velocity.

3. Apparatus for detecting the velocity of a moving vehicle comprisingan inductive loop of electrically conductive wire oriented with respectto a roadway such that a vehicle moving in a prescribed orientation withrespect to said loop induces a sinusoidally varying voltage in saidloop,

means for measuring the frequency of the initial portion of said inducedsignal voltage for producing an indication of the vehicle velocity,comprising first means for detecting the occurrence of one transition ofsaid induced signal voltage,

second means for detecting the occurrence of a transition of saidinduced signal voltage occurring subsequent to said one transition, and

means for measuring the time interval between the occurrence of said oneand said subsequent transitions for producing an indication of thevehicle velocity,

frequency selective means for coupling the induced signal voltage insaid loop to said first and second detecting means, said frequencyselective means having a low frequency pass characteristic with a highfrequency roll 011 of approximately 6 db per octave, and

means for converting the sinusoidal variations of the induced signalvoltage from said frequency selective means to one polarity with respectto a reference potential,

said first and second detecting means comprising third means responsiveto the output of said converting means for detecting the time ofoccurrence of the peak values of consecutive variations of the signalvoltage, said third detecting means comprising a comparator having firstand second input terminals and an output terminal,

first and second diodes connected in the same polarity between the firstand second input terminals, respec-

1. Apparatus for detecting the velocity of a moving vehicle comprising asingle passive sensor comprising an inductive loop of electricallyconductive wire oriented with respect to a roadway such that a vehiclemoving in a prescribed orientation with respect to said loop induces asinusoidally varying voltage in said loop, means for measuring thefrequency of the initial portion of said induced signal voltage forproducing an indication of the vehicle velocity, comprising first meansfor detecting the occurrence of one transition of said induced signalvoltage, second means for detecting the occurrence of a transition ofsaid induced signal voltage occurring subsequent to said one transition,and means for measuring the time interval between the occurrence of saidone and said subsequent transition for producing an indication of thevehicle velocity, frequency selective means for coupling the inducedsignal voltage in said loop to said first and second detecting means,said frequency selective means having a low frequency passcharacteristic with a high frequency roll off of approximately 6 db peroctave, and means for converting the sinusoidal variations of theinduced signal voltage from said frequency selective means to onepolarity with respect to a reference potential, said first and seconddetecting means comprising third means responsive to the output oF saidconverting means for detecting the time of occurrence of the peak valuesof consecutive variations of signal voltage.
 2. Apparatus according toclaim 1 wherein said time interval measuring means comprises a generatorresponsive to one output of said third detecting means for initiatinggeneration of a linear ramp voltage, and a circuit responsive to asubsequent output of said third detecting means for sampling and holdingthe magnitude of the ramp voltage, the amplitude of the ramp voltagebeing a measure of the vehicle velocity.
 3. Apparatus for detecting thevelocity of a moving vehicle comprising an inductive loop ofelectrically conductive wire oriented with respect to a roadway suchthat a vehicle moving in a prescribed orientation with respect to saidloop induces a sinusoidally varying voltage in said loop, means formeasuring the frequency of the initial portion of said induced signalvoltage for producing an indication of the vehicle velocity, comprisingfirst means for detecting the occurrence of one transition of saidinduced signal voltage, second means for detecting the occurrence of atransition of said induced signal voltage occurring subsequent to saidone transition, and means for measuring the time interval between theoccurrence of said one and said subsequent transitions for producing anindication of the vehicle velocity, frequency selective means forcoupling the induced signal voltage in said loop to said first andsecond detecting means, said frequency selective means having a lowfrequency pass characteristic with a high frequency roll off ofapproximately 6 db per octave, and means for converting the sinusoidalvariations of the induced signal voltage from said frequency selectivemeans to one polarity with respect to a reference potential, said firstand second detecting means comprising third means responsive to theoutput of said converting means for detecting the time of occurrence ofthe peak values of consecutive variations of the signal voltage, saidthird detecting means comprising a comparator having first and secondinput terminals and an output terminal, first and second diodesconnected in the same polarity between the first and second inputterminals, respectively, of said comparator and the output of saidconverting means, a capacitor connected between said second inputterminal of said comparator and a reference potential, and a third diodeconnected in the opposite polarity in parallel with said second diode.